Lithography is a commonly used technique for etching a desired pattern into a substrate. Typically, the substrate to be etched is secured into place with a vacuum chuck or other securing device opposite a radiation source. A lithography mask is mounted between the radiation source and the substrate. The lithography mask is formed of a material that is opaque to radiation produced by the source but includes a radiation-transparent window with an opaque pattern. When the radiation source is activated, radiation emitted therefrom travels through the transparent portions of the window to the substrate but is prevented from passing through the opaque portions. A photoresist on the substrate is exposed in the desired pattern by the radiation passing through the transparent portions of the mask window. The photoresist is then developed and then used as a mask to etch the substrate.
In the lithography imaging process, particularly for the production of microelectronic components, patterns recorded onto photo-resist covered semiconductor wafers should be precisely positioned relative to pre-existing structures. For example, in 1 X proximity lithography, variations of only a few nanometers over a pattern area of up to 50 millimeters.times.50 millimeters are permissible for acceptable pattern definition. Any stress present in the lithographic mask can distort the X-ray image and thus render the registration of the pattern with the substrate to be unacceptable. Such stress can be induced by many sources, including the actual gripping of the mask by its mounting structure. In particular, stress can be induced in the mask by a mismatch of the mask and its mounting structure. If the mask itself is not planar, or if the mounting structure includes mounting surfaces that are not coplanar, the mask may be required to deform in order to be secured. Often, sufficient deformation (and, as a result, stress) is induced in the mask to adversely affect imaging.
One solution to reducing or eliminating stress in a mask through the design of its mounting structure stems from recognizing that any rigid planar or nonplanar surface can be supported at three points without further deformation. In addition, the stress in a mask can be reduced by enabling three such support points to move in such a way as to comply with any inplane dimensional changes of either the mask or mask mount (such as those unused by thermal expansion) after the two are joined. These three points must be compliant, yet must constrain the mask sufficiently to maintain a precise location relative to a mounting device for the mask.
Mask mounts exist which attempt to address these design parameters, with the methods of holding and providing compliance differing between the mount designs. Designs for X-ray lithography mask mounts generally employ a mask-to-wafer gap of typically 10-50 micrometers. This gap precludes the use of any holding or clamping hardware from being located on the mask front (wafer side) surface. One mask mount design has a mask with a thin, radially outwardly projecting lip. This lip is located nearer to the mount than the plane defined by the mask front surface. The mask is attached to a cassette by three clamps, each of which pinches a portion of the mask outer lip to hold the mask in place. All clamps are 120.degree. apart and include a ball bearing that contacts the surface of the mask lip. In one clamp, the ball bearing is nested within an indented ball seat in the mask ring, and thus does not allow that mask mount point to move relative to the cassette. Another clamp is designed so that its ball fits within a linear V-groove channel on the mask lip. This clamp allows the mask to move relative to the cassette, in a direction defined by the V-groove. The third clamp is made so that its ball contacts on a flat surface on the mask lip, and thus is able to move in any direction parallel to the flat surface. The in-plane compliance achieved via the ball seat, V-groove and flat are referred to as a 1-2-3 kinematic mount. Once the mask is loaded within the cassette, the cassette is then attached to the stepper using a full surface vacuum chuck.
This design has certain shortcomings. The use of this mount is limited to masks that have a radially outwardly extending lip and the described ball grooves. This mask format is new and would require industry to forego existing mask formats and retool for this new format. In addition, finite element modeling has indicated that misalignment of the clamping pinchers could torque and distort the mask. This mask is more massive, which could lead to greater distortions due to gravitational sag; also, greater mask mass can negatively impact the performance of future mask/wafer aligners, which may have fast alignment stages which, in order to provide fast response time, should employ low inertia masks.
In a second design, the mask mount includes three metal vacuum cups, each of which pivots about a ball and socket joint. The mask is attached by suction to the rim of each cup. The cups are able to pivot in any direction to conform to the plane of the mask. However, their movement is not limited to directions parallel to the plane of the mask, as any pivoting by a cup distorts the attached mask out of its natural plane. In addition, because none of the cups are fixed into position, errors in remounting can lead to inconsistent imaging. Air leaks occurring at the metal cup rim will also generate vibrations, which distort the image.
In another design, a vacuum groove is machined into a metal plate. The mask ring and plate make full surface contact; the vacuum is applied to secure the mask to the plate. If nonplanarities exist between the mask ring and the vacuum plate, the ring will comply with the stiff plate surface and induce stress in the mask.